Newb Questions

Truck-Magnums have dual mounting pads, but the truck bosses, I am told, will interfere with header selection.
Per a recent discussion here on FABO, 67 to 72 headers, and Schumacher adapter-mounts on a slanty-K, do Not play well together.
IMO;
Your best bet is to find a 73up V8 A-body or similar vintage chassis, and transfer all the required bits over from it. The LA family is not to be feared. If you are opening the engine up anyway, then the only advantage might be the roller-cam, which to me is relatively meaningless.
Your slanty, besides a cid deficit, mostly suffers from a lack of rear gear and stall. For a city-only type combo, this is easy to get around/overcome.